Round, thin-walled, spirally grooved or ribbed metal ducts and their method of manufacture are known as evident by U.S. Pat. Nos. 3,435,852 and 3,621,884. Round ducts of this type have many uses, e.g., as cold or warm air ducts for residential, commercial, and industrial construction.
It is also known to reform at least one end of such ducts to facilitate joining two ducts together as evidenced by U.S. Pat. Nos. 3,708,867, 3,758,139, and 3,794,364. These methods involved rolling one end of the duct to flatten the spiral ribs sufficiently to permit the threading of the rolled duct end into an unrolled duct end, or rolling the ribs on both ends of the duct down with a narrow roller and then crimping one of the ends to make it small enough to fit into a rolled but uncrimped end. The problem with each of these methods is that the ends are fragile, particularly when the ducts are made from very thin metal, such as 15 mils or less, and particularly when aluminum metal is used. The fragile duct ends become damaged in shipment, handling, or installation rendering the ducts either useless or requiring expensive repair. As a result the tendency has been to use thicker gage metal to provide for stronger ends or to use heavier gage metal coupling members. Both alternatives are less than desirable.
The object of the present invention is to provide metal ducts having ribs with reformed ends that permit joining without auxiliary coupling members, which ends are much stiffer and significantly more resistant to damage than the prior art reformed ends.